An X-ray detector based on a group II-VI semiconductor such as CdTe or CdxZn1−xTe (0≦x≦1) is used in various fields including nuclear physics, X-ray and gamma ray astronomy, and medical applications. An important feature of such a detector is a uniform signal response over the whole detection region. However, the signal response sensitivity of a radiation detector obtained by dicing a wafer gradually increases or decreases close to the edges of the detector, and a reduction in performance is demonstrated close to the edges of the detector. This is known as the “edge effect or end face effect (edge effect)”, and is also reported in US Patent Laid-Open US 2011/0272589 A1 and A. Shor at al., “EDGE EFFECTS IN PIXELATED CdZnTe GAMMA DETECTORS”, IEEE TNS Vol. 51, No. 5, 2004 and M. Bosma et al., “THE INFLUENCE OF EDGE EFFECTS ON THE DETECTION PROPERTIES OF CdTe”, IEEE conference record 2011, for example.
The edge effect (or end face effect) may be attributed to distortion of the internal electrical field close to the edges or to very high surface leakage current, caused by defects produced as a result of unsuitable dicing or surface instability etc. This leads to a reduction in detection performance close to the edges, referred to as low charge collection efficiency or low energy tail structure in the photoelectric peak of the energy spectrum.
One method which is currently used to solve the edge effect (or end face effect) employs a guard ring. The guard ring is, according to a normal example, formed at the main surface peripheral edge region of a detector and is electrically connected, or floating or biased. By means of the guard ring, distortion of the internal electrical field close to the edges of the detector is reduced and the guard ring collects current so that side-surface leakage current is restricted. It is therefore possible to envisage an improvement in detector performance close to the edges. Until the present time, various kinds of guard ring structures in which a guard ring is formed on the main surface or side surface of a detector have been used in radiation detectors (US Patent Laid-Open US 2011/0272589 A1, U.S. Pat. Nos. 6,034,373 and 6,928,144; Nakazawa et al., “IMPROVEMENT OF THE CdTe DIODE DETECTORS USING A GUARD RING ELECTRODE”, IEEE 2004).
However, it may still not be possible to adequately restrict the edge effect (or end face effect) using a guard ring. This is believed to be due to unforeseeable defects on the side surface of the detector caused by unsuitable dicing or inadequate surface stability. In addition to using a guard ring, it is therefore important to reduce or restrict defects at the side surface of the detector. In order to eliminate such defects, side surface polishing or etching may be used for individual detectors, but it is difficult to control such processes and they are unsuitable for mass production in semiconductor wafer processes.
Unsuitable dicing produces mechanical defects at the side surface of the detector such as a large kerf width, and cracking or chipping which produces a rough side surface, and also results in a reduction in detector performance and a decrease in yield. A method of dicing a GaAs wafer that can restrict such mechanical defects is described in U.S. Pat. No. 5,182,233. U.S. Pat. No. 5,182,233 indicates that there is a clear difference in comparison with the mechanical cutting quality at the side surface of a semiconductor detector when a GaAs wafer is diced at the <110> and <001> crystal orientations. When a GaAs wafer is diced at the <110> crystal orientation, a large kerf width and chipping occur, and cracking and roughness are produced at the side surface of the detector. In contrast to this, when a GaAs wafer is diced at the <001> crystal orientation, the kerf width is reduced and chipping is restricted.
In the case of a group II-VI semiconductor wafer such as CdTe, no clear difference is apparent in the mechanical cutting quality at the side surface of a detector diced at the <110> crystal orientation and the side surface of a detector diced at the <001> crystal orientation, as mentioned above. There is thus a need for a dicing method which takes account of both reducing mechanical defects such as those mentioned above and reducing internal progressive defects which occur close to the edges of the detector and progress (spread) to the inside of the detector. The present invention relates to a group II-VI semiconductor radiation detector based on CdTe or CdZnTe, for example, and to a method for producing same; the present invention provides a radiation detector enabling a reduction or restriction of the edge effect (or the end surface effect) and a dicing method therefor, while taking account of the correlation between progression of defects and the slip system of a CdTe crystal, to give one example.